Various techniques for precisely measuring distance to objects or thicknesses of objects by optical means are known. These techniques include laser triangulation, conoscopic holography, low-coherence interferometry, chromatic confocal point sensing, frequency modulated continuous-wave (FMCW) laser radar, swept-frequency optical coherence tomography, and phase modulation range finding. (See, e.g., M.-D. Amann, et al., “Laser ranging: a critical review of usual techniques for distance measurement,” Opt. Eng. 40(1) 10-19 (January 2001), F. Blateyron, Chromatic Confocal Microscopy, in Optical Measurement of Surface Topography, (Springer Berlin Heidelberg) pp 71-106 (2011), C. Olsovsky, et al., “Chromatic confocal microscopy for multi-depth imaging of epithelial tissue,” Biomed Opt Express. May 1, 2013; 4(5): 732-740, G. Y. Sirat et al., “Conoscopic holography,” Opt. Lett. 10, (1985), W. C. Stone, et al., “Performance Analysis of Next-Generation LADAR for Manufacturing, Construction, and Mobility,” NISTIR 7117, May 2004, and M. A. Choma, “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Exp. 11 (18), 2183 (2003).) These techniques offer varying levels and combinations of measurement ranges, precisions, and resolutions.
Optical phase-sensitive detection techniques (also sometimes referred to as “coherent detection techniques”), such as low-coherence interferometry, optical coherence tomography and laser radar, can offer extremely high resolution, but face unique challenges in measuring diffusely scattering surfaces due to speckle, the far-field interference pattern arising from the multiple scattering centers of a diffuse reflector. For relative lateral motion (i.e. motion perpendicular to the laser beam propagation direction, and thus not a Doppler shift) between the laser beam and a rough surface (with roughness less than the system resolution), researchers at the National Institute of Standards and Technology (NIST) recently showed that speckle-induced phase variations place a “strong limit” on the achievable range uncertainty and precision using the FMCW laser radar technique. (See, E. Baumann, et. al, “Speckle phase noise in coherent laser ranging: fundamental precision limitations,” Opt. Lett., Vol. 39, Issue 16, pp. 4776-4779 (2014).) This reference (“Baumann”) is incorporated herein by reference in its entirety. The researchers showed that speckle noise resulting from surface roughness of a laterally moving surface (or laser beam) leads to a non-Gaussian range distribution with measurement errors that can dramatically exceed both the Cramer Rao lower bound and the surface roughness amplitude. Motion of the beam location on the sample surface degrades measurements significantly compared to the case where each successive point is measured statically, even to the point where outliers during lateral motion approach the system range resolution (given by c/2B, where c is the speed of light and B is the information bandwidth). As a result, the use of FMCW laser radar for high-precision surface imaging at a distance, for instance, is limited to either static point-by-point measurements, spatial averaging, or they must endure degraded precision when the beam location on the sample surface is in motion. Unfortunately, such lateral motion is needed for a variety of applications including non-contact, in-situ industrial metrology and impression-based forensics evidence. Baumann identifies the speckle noise problem with no solution. Solutions to the surface roughness speckle noise problem are therefore needed.